DELTA PSC (Delta Piscium). The constellation Pisces, that of the celestial Fishes, is so large, dim, and scattered that without the western "Circlet" it would be hard to recognize as any kind of singular figure. Buried in the heart of the southern extension to the southeast of the Great Square of Pegasus is near-anonymous fourth magnitude (but at 4.43 very close to fifth) Delta Psc. Obscure though it may be it has its own stories to tell. Classed as a cool, orange K (K5) giant, the star is much like Aldebaran (Alpha Tauri) though, at a distance of 311 light years (give or take 7), almost five times farther away. From a variety of measures, the temperature seems well-defined at 3975 Kelvin, which (to account for a lot of infrared radiation) with distance tells of a luminosity 380 times that of the Sun. To produce that brightness at that temperature requires a large size, calculated to be 42.1 times that of the Sun, or roughly half as big as the orbit of Mercury. Only two degrees north of the ecliptic, Delta Psc is regularly occulted by the Moon. The time it takes for the star to be covered by the easterly-moving lunar disk gives its angular size, which with distance and a bit of trig gives the physical size. As is often the case with large cool stars, the directly-determined radius depends on color, or wavelength. In blue light, the radius is 43.1 times solar, while infrared radiation gives 39.5 for an average of 41.2, satisfyingly close to that found from temperature and luminosity, showing that all the stellar parameters are in order and pretty accurately known. A slow projected equatorial rotation speed of 1.4 kilometers per second (similar to that of the Sun) yields a rotation period that could be as long as 1500 days, or just over four years. (Given that we have no idea about the axial tilt, it could be a lot less.) Theory then leads to a mass somewhere around 2-2.5 times that of the Sun (one source giving 1.5). Beginning life as a class A1 dwarf perhaps a billion or so years ago, the star is most likely brightening as a giant for the first time with a dead helium core. (Were it brightening for its second "ascent" as a giant with a dead carbon/oxygen core, it would probably be varying, at least slightly, and it isn't). Zipping along relative to the Sun at 55 kilometers per second (4-5 times normal), the metal content is consistently down by some 40 percent. Nearly two minutes of arc away lies a possible companion, 13th magnitude Delta Piscium B. If really bound to Delta A (which it may be as it's held its position rather well for the past century), from its brightness it is a K9 dwarf with a mass roughly half that of the Sun. At a separation of at least 12,500 Astronomical Units it would take at least 900,000 years to make a full orbit. More likely, especially given the fragile nature of any such gravitational bond, it is probably just another line-of-sight coincidence.
Written by Jim Kaler 1/06/12. Return to STARS.